Photovoltaic (PV) power conditioning systems usually include a maximum power point tracker (MPPT) in order to ensure that the maximum available power is extracted from the PV panels. The PV power conditioning system usually consists of two stages. The first stage is a DC/DC converter. The second stage is usually a DC/AC inverter. The DC/DC converter is responsible for providing galvanic isolation as required by grid interconnection regulatory standards. The DC/DC converter also boosts the voltage at the output of the PV panel. As well, the DC/DC converter control system is responsible for extracting the maximum power from the PV panel. The DC/AC converter converts the DC power to AC power which can be delivered to the utility grid.
FIG. 1 shows an exemplary arrangement of a two-stage
PV power conditioning system. In FIG. 1, illustrated are: a PV power conditioning system 1, a PV-panel 2, a DC/DC converter 3, a DC-link 4, a DC/AC converter 5, and a utility grid 6. The DC/DC converter 3 performs the maximum power point tracking of the PV-panel 2. Also, present is an intermediate DC-link 4 between the two stages of the power conditioning system. This component is used so that energy can be stored and to provide decoupling between the first-stage and the second-stage of the system. As well, the DC-link 4 attenuates the double frequency ripple caused by the power ripple at the output of second-stage DC/AC converter 5. The DC/AC converter 5 produces power compatible to the utility grid 6.
The arrangement in FIG. 2 shows an exemplary arrangement of the DC/DC converter control system. This figure shows that the control DC/DC converter control system 30, includes: maximum power point tracker (MPPT) 32, and DC/DC converter controller 34. The MPPT 32 produces the reference value for the PV voltage/current according to the feedback signals. These feedback signals are the PV output voltage vPV and the PV output current iPV. The MPPT 32 requires both the PV output voltage and current in order to find the “maximum power point”. With the PV output voltage vPV, and the PV output current iPV, the MPPT 32 produces the reference value for either the PV output voltage, V*PV, or the PV output current, i*PV. This reference is the input to the DC/DC converter controller 34. The DC/DC converter controller 34 produces appropriate gate pulses for the DC/DC converter in order to regulate the converter input voltage or the converter input current.
In order to measure the output current of the PV panel, a current sensor able to measure direct current (DC) is required. Two types of current sensors are commonly used to measure this DC current: Hall-effect current sensors, and resistive current sensors.
Hall effect sensors suffer from several practical difficulties. Due to the remnant flux, such Hall effect sensors introduce a time varying DC-bias into the control system. A correction algorithm therefore has to be added to compensate for this time varying DC bias. This correction algorithm increases the complexity of the implementation of the control system and reduces the reliability of the converter. As well, the bandwidth of the Hall effect sensors are limited and they introduce delay into the closed-loop control system. Such delay may jeopardise the stability of the control system. Finally, Hall effect sensors are very costly and can significantly contribute to the overall cost of the converter.
The second technique to measure the input current is through the use of resistive current sensors. Resistive current sensors require a very precise and noise-free differential amplifier. These types of sensors also increase the power losses of the converter. These power losses are not preferable due to the efficiency deterioration and thermal management. The arrangement in FIG. 3 shows an exemplary arrangement of the resistive current sensor used to measure the output current of a PV panel. Referring to FIG. 3, illustrated are: a resistive current sense 36, a precise difference amplifier 38, and an analogue-to-digital converter (ADC) 40. This circuitry in FIG. 3 produces the digital value of the PV panel output current for the MPPT 32. According to FIG. 3, the resistive current sensor requires a precise difference amplifier in order to accurately measure the current. Also, at light loads the accuracy of the resistive current sensor is highly compromised due to the small value of the current sense resistor Rsense. Therefore, at light loads the performance of MPPT is significantly deteriorated.
For multi-input PV power conditioning systems, in particular, the input current sensors add a lot of complexity and cost to the power conditioning system. The arrangement in FIG. 4 shows an exemplary arrangement of a four-input PV power conditioning system. This power conditioning system is able to perform maximum power point tracking on each individual panels. However, the power conditioning system is required to measure each PV panel's output current in order to perform maximum power point tracking. The power conditioning system therefore requires current sense resistors, difference amplifiers and ADCs for each individual panel. Because of this, current sensors significantly contribute to the overall cost of the power conditioning system.
In addition to the above, there is another difficulty related to using resistive current sensors for multi-input power conditioning systems. The arrangement in FIG. 5 shows the multi-input PV power conditioning system with resistive current sensors. In order to minimize the required isolation circuitry for the DC/DC converter, the control system should have the same ground as the control system. This is because gate pulses produced by the control system can be directly applied to the DC/DC converters power switches without isolation. However, in this arrangement, the multiple inputs of the power conditioning system cannot operate in parallel, since the current sense resistors will be in parallel and cannot indicate the precise value of the current for each panel. This difficulty restricts the application of the multi-input PV power conditioning systems for different applications. In particular, this arrangement cannot be applied to applications where the PV panels are arranged in parallel.
There is therefore a need for a simple and practical solution which can provide the output current information for the PV panel. Preferably, such a solution should not add extra circuitry to the power conditioning system. It would also be preferable if such a solution can provide the current information very precisely and reliably under different conditions.